This invention relates generally to techniques for producing interconnecting conductive links and, more particularly, to a unique process for providing conductive links between two conductive materials having a non-conductive material positioned between them.
Configurations of interconnected arrays of conductive elements, as used, for example, in programmable logic gate arrays, requires the formation of conductive links, or paths, between selected conductive elements in a manner which produces relatively low resistance links between them. Techniques for producing such low resistance conductive links have been developed using either electrical or laser linking and cutting processes.
The latter laser processes have been preferred for certain applications because they provide permanent links and require no programming wiring or high voltage switching on the chip. Laser programmable techniques have the potential for providing higher performance and greater link density than electrical techniques if the linking device itself is sufficiently small. Ultimately the minimum size laser link would be a simple crossing of two wires. However, up to now insofar as is known, a successful process does not exist for providing such links. A primary concern when using any linking technology is the ability to use standard processing for the metal lines on the insulation. More specifically, this means the ability to integrate laser restructurable elements using standard silicon based MOS processing without the need to incorporate additional steps. Lateral links, which produce conductive links using silicon diffusion, have been used for some time to achieve compatibility with CMOS processing, as disclosed in U.S. Pat. No. 4,455,495, to Masuhara et al. and in U.S. Pat. No. 4,937,475, issued to F. M. Rhodes et al. on Jun. 26, 1990. These techniques require large areas to focus the laser to the substrate and have high resistance.
Other recent exemplary techniques have been proposed using laser linking processes for interconnecting metal layers at different levels. One such technique is disclosed in U.S. Pat. No. 5,166,556 issued on Nov. 24, 1992 to F. Shu et al. in which a laser beam is applied to an upper titanium metal layer at the location at which a link is desired to be made with a lower titanium layer. Laser power is supplied at a sufficient level to cause a chemical reduction reaction between the titanium layers and the intermediate silicon dioxide insulating layer so as to produce a conductive compound between the titanium layers which acts as an electrically conductive circuit therebetween. Such technique requires additional non-standard process steps and produces high resistance links and, hence, low performance.
U.S. Pat. No. 4,810,663 issued to J. I. Raffel et al. on Mar. 7, 1989 discusses a technique in which a diffusion barrier layer is placed between each metal layer and the insulation layer and the link region is exposed to a low power laser for a relatively long time (i.e., a relatively long pulse width) to cause the metals to alloy with the diffusion and insulating layers to form the desired conductive link. Such technique requires a relatively long laser power pulse output using a relatively complicated diffusion barrier/insulation structure so as to produce an opening in the upper layer to permit the energy to be applied to the barrier and insulating layers to produce the desired alloying operation.
A further technique has been proposed to provide lateral links between metals substantially at the same surface or plane as discussed in U.S. Pat. No. 4,636,404 issued to J. I. Raffel et al. on Jan. 13, 1987. Again relatively long pulses are applied to the general region between the metals so as to cause the metals to form an aluminum doped silicon link.
In a recent article xe2x80x9cLaserpersonalization of Interconnection Arrays for Hybrid ASIC""s of M. Burnus et al., IEEE International Conference on Wafer Scale Integration, 1993, a laser beam is used to provide sufficient power to blast a hole through an upper metal layer so as to form an opening at the link region. Multiple laser pulses of high energy density are used to create the opening and to remove the insulating layer between the metal elements. The multiple pulses also produce molten aluminum which spreads along the walls of a crater that is formed when the insulating layer is removed beneath the opening. Such aluminum flow along the crater walls produces a tube-like aluminum contact body between the upper and lower aluminum layers.
The article xe2x80x9cLaser Programmable Vias for Reconfiguration of Integrated Circuitsxe2x80x9d by Rouillon-Martin et al. in Optical Microlithography and Metrology for Microcircuit Fabrication, 1989, discloses a technique which performs a similar operation to that discussed in the above Burnus et al. article in which the opening is made much smaller in diameter by using multiple pulses of a relatively highly focused laser beam.
It is desirable to devise a laser linking process which produces a link structure between any two metal layers which can be fabricated in a manner which is compatible with standard MOS processes and which provides high performance (low resistance) and high density (small area) links. Such process should use relatively low laser power and provide self-contained links with low peripheral damage at the link sites.
In accordance with one embodiment of the invention, in a linking process an energy producing device, e.g., a laser, applies a single pulse of sufficient energy to at least one of two conductive materials which are to be linked, and which have a non-conductive material between them, so as to produce mechanical strain in at least one of the conductive materials. The strain that is produced initiates a fracturing of the non-conductive material so as to provide at least one fissure therein which extends between the conductive materials. The single energy pulse applied by the energy producing device further causes a portion of at least one of the conductive materials to flow in the at least one fissure to provide at least one conductive link between the conductive materials. In most cases, an effective fissure extends from a point at or near an edge of at least one of the conductive materials to the other conductive material.
Preferably the non-conductive material is a silicon based dielectric such as silicon oxide or silicon nitride. The fracture and conductive link is preferably obtained with a pulse of energy of less than 1 microjoule and a pulse duration of less than 1 microsecond. The most preferred pulse duration is in the range of 1 to 10 nanoseconds.
In the preferred embodiment, the first and second conductive materials are metals lying substantially in the same plane and the fissure extends generally laterally between the first and second conductive materials. By forming the non-conductive material in layers, an interface between the layers can control the fracturing. The fracture may be formed along or be limited by the interface. Preferably, an interface of hard material over softer material is provided at a level above the conductive materials.
In another embodiment of the invention, an upper metal layer is deposited on the non-conductive material in a manner so as to provide a preformed opening at the desired link site. A single pulse of energy can then be used to be effectively applied to the lower metal layer at the link site so as to produce the mechanical strain required to initiate the fracturing of the dielectric or insulating material, as discussed above. In a still further embodiment of the invention, if a preformed opening is used in the upper metal layer, a single laser pulse of energy may be used to provide a desired chemical reaction, or desired alloying, or a desired removal of the dielectric to create a crater therein, without having to produce an opening through the upper metal layer. Accordingly, by the use of such a preformed opening a conductive link may be formed from fracturing or from a chemical reduction reaction process, an alloying process, or the flow of metal in a crater formed in the dielectric which has been removed at the link site. Thus, in some cases the use of the preformed opening may not require a fracturing of the dielectric material between the metal elements.
In accordance with a preferred implementation of the invention, a first pattern of first preformed conductive elements and a second pattern of second conductive elements are provided on a substrate and lateral conductive links are formed at a single level between selected ones of the first and second conductive elements so as to provide a plurality of desired conductive paths. As used herein, the term xe2x80x9clevelxe2x80x9d is intended to include embodiments wherein the surfaces of the elements involved in the link lie at substantially the same level, whether or not the elements themselves are formed or deposited on the same planar surface. In one embodiment thereof, for example, a first pattern of conductive elements is formed at a first level while a second pattern of second conductive elements is such that it has portions of its conductive elements at two levels, i.e., first portions thereof being at the same level as the first conductive elements and other portions thereof being at another second level below the first level. Lateral conductive links are then formed at a single level between selected ones of the first conductive elements and selected ones of the portions of the second conductive elements that are at the same level. In another embodiment, for example, the first and second patterns of conductive elements are all formed at the same level and lateral links are formed at such level. In addition, appropriate cuts can also be made to separate, as necessary, the conductive paths that are so formed.